1. Field of the Invention
The present invention relates to a slit lamp microscope for observing a patient's eye (an examinee's eye) while irradiating a treatment laser beam to the patient's eye, and an ophthalmic laser treatment apparatus provided with the slit lamp microscope.
2. Description of Related Art
In an ophthalmic laser treatment apparatus arranged to irradiate a treatment laser beam (a laser beam for photocoagulation) to a patient's eye, particularly, a fundus of the eye, a binocular slit lamp microscope is used for observation. As such a slit lamp microscope, there is a configuration that comprises a variable magnification mechanism including a rotating drum with a variable magnification optical system placed within a binocular observation optical system for changing observation magnification (power) by rotating the rotating drum (see for example JP2002-224036A, hereinafter referred to as “Patent document 1”). Besides, there is also a configuration in which an optical element called Stereo-Variator for changing a viewing angle is installed between an objective lens and a variable magnification mechanism in order to adjust a binocular field of view during observation (see for example U.S. Pat. No. 4,838,671 (JP62-269923(1987)A), hereinafter referred to as “Patent document 2”).
In the apparatus of Patent document 2, however, a mechanical part of the stereo-variator is about 70 mm in length, resulting in an increased length of an observation optical system. Accordingly, the microscope has an extended working distance (herein, corresponding to a distance from eyepieces for observer (operator) to a patient's eye). In the photocoagulation laser treatment of a fundus of the patient's eye, an operator holds a contact lens on the patient's eye with hand and simultaneously performs laser irradiation using illumination light and treatment light delivered to the patient's eye. If the working distance is long, therefore, some operators who have short arms may not stably hold the contact lens in contact with a patient's eye while observing the patient's eye. Further, the mechanical part of the stereo-variator in Patent document 2 is large in size and needs a rotating mechanism for largely changing the viewing angle, leading to a high cost.